An unstable nucleus performs nuclear disintegration many times to be a stable nucleus. In this case, the nucleus emits alpha-ray, beta-ray, and gamma-ray with specific energy.
A radionuclide detector is an apparatus which measures and analyzes energy of radiation generated from a substance including an unknown radionuclide to distinguish a unique radionuclide constituting the substance.
A scintillator used in a radionuclide detector has a property of emitting light when receiving energy. By measuring the amount of light generated by radiation, it is possible to confirm energy of the incident radiation. When the scintillator receives radiation with higher energy, the scintillator generates a larger amount of light. For example, polyvinyl toluene (PVT) scintillator generates 10,000 photon/MeV.
As radionuclide detectors, there are a detector using a plastic scintillator and a detector using an inorganic scintillator.
In a case of the scintillation detector using an inorganic scintillator (for example, NaI(TI) or CsI(TI)), a ratio in which a photoelectric effect of absorbing all energy of gamma-ray occurs is high, and energy resolution is excellent. Accordingly, it is possible to easily analyze a radionuclide on the basis of energy of a photo peak. However, since costs are high, it is difficult to be applied to a large-scale detector.
In the radionuclide detector using a plastic scintillator (or another kind of organic scintillator), a photoelectric effect rarely occurs, and energy resolution is relatively low. Conventionally, an energy window has been used in which only the amount of radiation transferred to a detector is measured using a gross counting method, or in specific field, several energy areas corresponding to target nuclides are set to interest areas and the amount of radiation measured in each area is evaluated to analogize nuclides. In other words, a group including nuclides with similar energy has been analogized rather than analyzing nuclides themselves.
This method has many limits due to restrictive energy resolution and statistic error when energy difference between nuclides is not large. For example, a radiation monitor for detecting inflow of illegal radioactive substances at the airport or harbor needs a large-area detector. However, a conventional radiation monitor using a plastic scintillator does not distinguish 137Cs and 214Bi (0.662 MeV and 0.609 MeV, respectively) which are an artificial radionuclide and a natural radionuclide, and does not distinguish 60Co and 40K (1.332 MeV and 1.505 MeV, respectively). In this case, flow of distribution is disturbed, and also a harmful effect in economical aspect is significant such as causing loss of manpower due to additional secondary inspection.
In the present invention, a method and an apparatus for distinguishing radionuclides by applying a weight to energy spectrum of radiation obtained using a plastic scintillator are proposed. In addition, this method is simulated by Monte Carlo simulation to verify the effect.